Fair Value Measurements - Financial Assets or Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair value measurements, recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Short-term investments:
Total assets at fair value	$ 50,785	$ 80,517
Short-term investments | Corporate commercial paper, stock, bonds and notes
Short-term investments:
Corporate commercial paper, stock, bonds and notes	32,684	49,100
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	16,521	13,568
Corporate commercial paper, bonds and notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,250	9,999
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	330	7,850
Quoted Prices in Active Markets (Level 1)
Short-term investments:
Total assets at fair value	16,521	18,213
Quoted Prices in Active Markets (Level 1) | Short-term investments | Corporate commercial paper, stock, bonds and notes
Short-term investments:
Corporate commercial paper, stock, bonds and notes	0	2,644
Quoted Prices in Active Markets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	16,521	13,568
Quoted Prices in Active Markets (Level 1) | Corporate commercial paper, bonds and notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		2,001
Other Observable Inputs (Level 2)
Short-term investments:
Total assets at fair value	34,264	62,304
Other Observable Inputs (Level 2) | Short-term investments | Corporate commercial paper, stock, bonds and notes
Short-term investments:
Corporate commercial paper, stock, bonds and notes	32,684	46,456
Other Observable Inputs (Level 2) | Corporate commercial paper, bonds and notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,250	7,998
Other Observable Inputs (Level 2) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 330	$ 7,850